1. Field of the Invention
The present invention relates to a carrier for electrophotography, that composes together with a toner a two-component type developer for developing electrostatic images, and a process for producing the carrier.
The present invention also relates to a two-component type developer for developing electrostatic images, comprised of a toner and a carrier.
The present invention still also relates to an image forming method for developing a latent image by the use of a two-component type developer comprised of a toner and a carrier, under application of a bias voltage in a developing zone.
2. Related Background Art
As conventional electrophotography, various methods are disclosed, for example, in U.S. Pat. No. 2,297,691, Japanese Patent Publication No. 42-23910 (U.S. Pat. No. 3,666,363) and Japanese Patent Publication No. 43-24748 (U.S. Pat. No. 4,071,361). In all of these methods, copies are obtained by irradiating a photoconductive layer with an optical image corresponding with an original to form an electrostatic latent image, subsequently adhering onto the electrostatic latent image a colored fine powder, called a toner, having a polarity opposite to it to develop the electrostatic latent image, and transferring the toner image to a transfer medium such as paper if necessary, followed by fixing by the action of heat, pressure, or solvent vapor.
The step of developing the electrostatic latent image is a step in which toner particles statically charged to a polarity opposite to the latent image is attracted by electrostatic attraction force so as to be adhered onto the electrostatic latent image. (In the case of reversal development, a toner having triboelectric charges with the same polarity as the charges of a latent image is used.) In general, methods of developing such an electrostatic latent image by the use of a toner can be roughly grouped into a method making use of what is called the two-component type developer, comprising a toner dispersed in a small amount in a medium called a carrier, and a method making use of what is called the one-component type developer, comprising a toner alone without use of a carrier.
In general, the carrier that composes such a two-component type developer can be roughly grouped into a conductive carrier and an insulative carrier.
The conductive carrier is usually comprised of oxidized or unoxidized iron powder. A developer composed of this iron powder carrier, however, has the problem that the triboelectric chargeability to a toner is so unstable that fog may be generated on a visible image formed using the developer. More specifically, as the developer is used, toner particles are adhered to the surfaces of the iron powder carrier particles, so that the electrical resistance of carrier particles increases to lower bias currents, and also to make the triboelectric chargeability unstable, resulting in a lowering of the image density of a visible image formed and an increase of fog.
The insulative carrier is commonly typified by a carrier comprising carrier core particles comprised of a ferromagnetic material such as iron, nickel or ferrite whose surfaces are uniformly coated with an insulating resin. A developer that employs this carrier may cause little the fusing of toner particles to the carrier surfaces, compared with the case of the conductive carrier, and hence has the advantage that it is particularly suitable for high-speed electrophotographic copying machines in view of its superior durability and long lifetime.
However, the charges (or the quantity of triboelectricity) of the developer making use of such a carrier coated with an insulating resin commonly tend to change with variations of environmental conditions such as low temperature and low humidity and high temperature and high humidity. As a result, for example, under conditions of low temperature and low humidity a lowering of image density is caused by charge-up, and under conditions of high temperature and high humidity the problems of fog and black spots around images are brought about because of a lowering of triboelectricity.
Thus, under existing circumstances, a carrier having reached a satisfactory level has not been discovered in regard to the carrier coated with an insulating resin.
As to a carrier coated with no insulating resin, various attempts have been made. For example, Japanese Patent Application Laid-open No. 62-229256 discloses a carrier comprising ferrite particles the surfaces of which a water-soluble quaternary ammonium salt is adhered to. Use of the water-soluble quaternary ammonium salt, however, has caused the disadvantage that the quaternary ammonium salt on the ferrite particle surfaces is dissolved out or eliminated after a toner has been left standing for a long period of time under conditions of high temperature and high humidity or after duration for copying (i.e. running), so that the properties of the particles gradually become close to the properties of untreated ferrite particles. In addition, since the particles are not coated with a resin, the quaternary ammonium salt on the ferrite particle surfaces tends to be eliminated not only after running under conditions of high temperature and high humidity but also after that in a normal environment of normal temperature and normal humidity. Even if the quaternary ammonium salt is not eliminated, there has been a problem after all, when compared with the resin-coated carrier, which is the problem that a toner forms a film on the surface of the carrier, i.e., the problem that a toner is so susceptible to the toner-spent phenomenon that a developer has a short lifetime. In addition, unless the particles are coated with a resin having insulating properties up to a certain degree, even in the case of ferrite particles as well as in the case of iron oxide powder, the leak of current occurs in a developing system in which a bias voltage is applied or the adhesion of carrier onto a photosensitive member is great. Thus, in respect of the durability and toner-spent resistance of carriers, no method is presently available which can be superior to the coating of the carrier with an insulating resin.